Many optical devices and systems may require polarized light for operation. Such devices may be found within diverse application areas, including for example flat-panel-displays, projection displays, optical fiber networks, and/or optical sensors. However, many light sources may be unpolarized, including for example light-emitting-diodes (LED), cold-cathode-fluorescent-lamps (CCFL), incandescent lamps, and/or natural light.
Polarizing elements, such as sheet polarizers or various birefringent prisms, can be used to convert unpolarized light to polarized light. However, such polarizing elements can be inherently lossy, since they typically operate by absorbing unwanted light or by redirecting the unwanted light away from a desired direction. This can result in greater than about 50% loss of optical power, even before the light enters the display component. Such large losses are typically undesirable, especially in high brightness display systems or portable battery-powered display systems where battery life is limited.
Some approaches have been used to reduce losses in polarizing elements so that the amount of unpolarized light that is converted to polarized light (referred to herein as “conversion efficiency”) is greater than about 50%. One such approach selectively passes the desired polarization into the display and reflects the unwanted polarization back into the illumination system, with the expectation that the unwanted polarization will be scrambled or converted into desired polarization and subsequently reemitted with at least some of the desired polarization. Such an approach may preserve the etendue (or extent of spreading) of the light output from the light source.
Another approach to polarization conversion, referred to as a polarization conversion system (PCS), converts the incident light with the unwanted polarization into the desired polarization, instead of absorbing or redirecting it from the output, and is described for example in U.S. Pat. Nos. 5,995,284 and 5,986,809. This can lead to polarization conversion with typically 60-80% efficiency.
One difficulty with the PCS approach may involve the polarization-separating element. In particular, many approaches may employ an array of small polarizing beam-splitters (PBS array). Such an array may be used satisfactorily for smaller incidence angles, but can experience substantial degradation for light incident off-axis at angles of about ±5° or more. An alternative PCS described in U.S. Pat. No. 6,621,533 employs a complex combination of a blazed micro-prism array with bulk liquid crystals; however, such a complex structure may be difficult to fabricate and/or may have performance limitations.
In addition, one or more polarization gratings (PGs) have been used in combination with a mirror or a waveplate and micro-prism array to achieve polarization conversion. However, the former arrangement may require a relatively large volume, while the latter arrangement may lack practicality for all but extremely collimated light.
Also, an arrangement of two PCSs has been demonstrated that employs traditional refractive microlens arrays, a polarization grating, and a louvered wave plate. However, such configurations may present challenges with respect to fabrication and registration across relatively large areas, as they may involve at least five elements, which much be fabricated separately and carefully aligned. Furthermore, the practical limitations the size and f-number (the ratio of the focal length to the diameter of a lens) possible for microlens arrays often limit performance or increase cost.